The best way to do this would be to set the alpha for the buttons on the keyboard. It would help if we knew if you are making your own keyboard or trying alter the built-in keyboard. Altering the built-in keyboard is out of the range of possibilities, unless you decide to root your device and replace the code for the keyboard with your own. If you are making your own keyboard, the only difference between the regular keyboard and a transparent one is where you set the alpha (transparency) for the buttons and layers.